The present invention relates generally to apparatus and methods for measuring the flow rate of a fluid and is applicable to probe and in-line apparatus for making such measurements. More specifically, the invention relates to thermal pulse flow measurements in which a relatively small bolus of flowing fluid is heated or cooled and the time required for the bolus to move downstream a known distance is measured.
Prior art thermal pulsed flow meters typically produce a heat pulse by placing an electric resistance heater in the flowing fluid. This raises the temperature of a small amount of the fluid that is then detected downstream by one or more temperature sensors. By measuring the time interval between the thermal pulse and the detected temperature increase, the fluid flow rate can be determined. The relatively long time required for temperature sensors to respond to the fluid temperature rise relegates this method to measurement of low fluid flow rates. In addition, the temperature sensor elements used are usually constrained to be relatively small and fragile in order to reduce response time. These are often not suitable for direct exposure to fluids in general purpose flow measurement applications.